Heap walker reference view - Reference graph

The reference graph list is one of the view modes in the reference view of the heap walker.

The reference graph shows the incoming and outgoing references of all instances of classes and arrays which are contained in the current object set. There is always one instance visible at a time. Above the upper right corner of the table, navigation controls allow to move back and forth through all the instances or arrays in the current object set.
The order of the instances in the object set can be adjusted to
- unsorted
  The objects are in a random order. This is the default setting.
- sorted by shallow size
  Objects with a larger shallow size are displayed first.
- sorted by deep size
  Objects with a larger deep size are displayed first.
After changing the sort order, the displayed index is set to one.

The instance navigation is linked with the data view of the heap walker to allow you to easily switch back and forth between these views while keeping the focus on the same object.

No specific view settings apply to the reference graph.

The references graph has the following properties:
- Instances are painted as rectangles with the class name of the instance written inside the rectangle.
- References are painted as arrows, the arrowhead points from the holder toward the holdee. If you move the mouse over the reference, a tooltip window will be displayed that shows details for the particular reference.
- The current instance has a violet background. In the current instance, the shallow size as well as the deep size of the object are shown.
- Garbage collector roots have a red background.
- String values are shown directly in the java.lang.String instance rectangle.
By default, the reference graph only shows the direct incoming and outgoing references of the current instance. You can expand the graph by double clicking on any object. This will expand either the direct incoming or the outgoing references for that object, depending on the direction you're moving in. Selective actions for expanding the graph are available in the view-specific toolbar and the context menu:
- Show outgoing references
- Show incoming references
If applicable, an instance has plus and minus signs at the left and the right side to show or hide incoming and outgoing references. The controls at the left side are for incoming, the controls at the right side for outgoing references. The plus signs have the same effect as the Show outgoing references and the Show incoming references actions. A minus sign hides all outgoing references and all objects that are not connected to the central instance. This can have the effect that the object on which you click the minus sign is hidden as well.

Additionally, the plus and minus signs give you the following indications:
- plus sign
  There might be references to display. You have not yet tried to expand them.
- minus sign
  You have expanded all references, there are no more references to expand.
- no sign
  You have tried to expand references, but there were none.
To reset the graph to its original state, you can choose Reset graph from the context menu.

The reference graph offers a number of navigation and zoom options.

To check why an instance is not garbage collected, you can select it and use the Show paths to GC root button in the view-specific toolbar or the corresponding entry in the context menu.
A dialog will ask you whether to search for a single garbage collector root or for all roots. After that, the paths to root are searched. This is a computationally expensive operation and can take some time. A progress dialog is shown while the paths to root are calculated.
- If the object is not referenced by a garbage collector root, a message box will be displayed. Note that this case is only possible if the "Remove unreferenced and weakly referenced objects" option in the heap walker option dialog is unchecked.
- Otherwise the graph is then expanded up to the garbage collector roots that were found. The garbage collector roots themselves are displayed with a red background.

There are three layout strategies for showing the reference graph which can be chosen by clicking on in the toolbar or choosing the layout strategy from the context menu.
- Hierarchic layout
  Standard layout that tries to layout the graph from left to right. This is suitable for most purposes.
- Organic layout
  Layout that tries to layout instances for optimal proximity. This layout is suitable for complex situations and can visualize clusters.
- Orthogonal layout
  Layout that tries to layout instances on a rectangular grid. This layout is suitable if your objects form a matrix.

To add a selection step from this view you can
- select one or multiple objects and click the [Use ...] button above the graph and choose selected objects in the popup menu. There is a corresponding entry in the context menu. A new object set will be created that contains only the selected instances. Multiple objects are selected by keeping the SHIFT key pressed during selection.
- select an array of objects or a standard collection from the java.util package and click the [Use ...] button above the graph and choose items in selected collection in the popup menu. There is a corresponding entry in the context menu. A new object set will be created that contains the objects in the array or collection. If you select a map collection, you are prompted if you want to include the key objects as well.
After your selection, the view helper dialog will assist you in choosing the appropriate view for the new object set.